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igbts for a small half bridge

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Ben Solon

Sun Jun 26 2011, 12:27AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

hello people, i am starting a drsstc that uses a half bridge of igbts. here is the tc design i am using. pretty simple. the IGBTs used where to-247 with (40A Icm) rating and (20A@25 degrees C) the guy who designed this pushed the ratings 5 times over the max limits...

anyways, back to the question: what should i look for in an igbt to build this design. i plan on doubling the Icm rating and keeping the 1200v rating. because this is my first drsstc i don't know the characteristics to look for. i want to get for for under 40$ however. the case type should be to-247 or the igbt "bricks"

thanks for the help,
ben

Dr. ISOTOP

Sun Jun 26 2011, 03:37AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

If you use a 500 kHz secondary, you will need *fast* IGBTs (HGTG40N60A4D speeds; essentially you'll need a 4th generation fast IGBT).
You definitely don't need the 1200V rating; 600V is enough for doubled mains operation - just be sure TVS and snub copiously.
If you try hard enough (actually, not that hard at all unless you are going for a *tiny* coil) you can use these:

Or just skip the small stuff and use CM400 half-bridge modules, which show up on Ebay for $15 a piece quite often. Only hard part about using bricks is they are slow (they become real sad as you push them to 60 kHz or so) and they have uncomfortably high gate charge (it is difficult to use a GDT to drive them in coil duty).

Ben Solon

Sun Jun 26 2011, 03:24PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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OK, i think i'll use the HGTG30N60B3D's with some tvs diodes. will the diodes alone be enough without a snubber though? i plan on running this on full wave rectified ac in the us, so thats 170vdc. if i can, il get 340vdc from 170v peaks. i dont plan on ever having more than 350v on the bridge. will just the tvs diodes be suficient?

one other thing. where can i get some ixydd414's? i googled them, and not even a datasheet.

thanks for the help,
ben

Dr. ISOTOP

Sun Jun 26 2011, 08:54PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

A snubber capacitor is always a good idea, even if you are not running a high bridge voltage. Any 600V film cap of reasonable capacitance will do.
You don't need IXDD414's; UCC37322's will suffice, are more than adequate, and can be sampled from TI/purchased cheaply on Ebay.

Ben Solon

Mon Jun 27 2011, 12:29AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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ok, thats about it then regarding the bridge. one last thing however. for the snubber and tvs, just confirming what to do: tvs diode rated for around 500v(leaving a safety margin) reverse breakdown in antiparallel to the igbt. snubber cap on rated for 600v(what range can i use? .1u? 1u? 10u? any resistor needed?) also in parallel to each igbt's drain and source.

anyways, thanks for all the help

Dr. ISOTOP

Mon Jun 27 2011, 02:10AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Snub across the entire half-bridge, the bigger the better (but make sure the cap is a cap of reasonable quality!)
Assemble the bridge, then scope it for spikes over 600V.

Arcstarter

Mon Jun 27 2011, 05:10AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

A snubber does not go across the drain and source of a mosfet. That would not end well for your mosfets...! A snubber goes across the supply. 10uf would be reasonable. Less may be fine, it depends on the leakage inductance. Like Bwang said, scope across the voltage supply right at the pins on the fets. If there is alot of ringing, more capacitance is needed (or the quality of the capacitor is not good enough) Low ESR and ESL is important, and the most suitable cap would be a film cap.

A snubber capacitor is also called a decoupling capacitor:

While i am on that subject, i should also say that there are some capacitors that should be used that is not included on that schematic. Decoupling capacitors should be used on the supply pins of all ICs as close as possible to the pins. Steve Ward's schematics include these capacitors:

(caps C2, C4, C5, and C14).

Have you ever made a sstc before? If not you may considering starting with one instead of a drsstc, it will put many things into perspective.

Dr. ISOTOP

Mon Jun 27 2011, 07:46AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

I too would recommend trying something small and H-bridge based before going on to a full-blown DRSSTC.
"Small" in this case doesn't mean wimpy and uncool; lots of things will run off a bridge of TO-247 IGBTs.
Also, on a completely unrelated note, if you are using TO-247's for DRSSTC duty, give each IGBT an isolated floating heatsink.
Also, there is no reason not to use a full bridge as opposed to a half-bridge...those Ebay IGBTs are cheap cheap cheap!
Also, don't drive your gates to 30V; 24V is enough unless you are doing ridiculous things with your transistors.

Ben Solon

Fri Jul 01 2011, 04:33PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

oh... decoupling... i am more used tho that term. i know it runs on dc though(i wasn't going to run it on on smoothed dc! or did u think i was going to try to feed it ac :0) i know all about h bridges however, so dont go and tell me to go do something simpler... the only thing i am concerned about is the gdt into the igbt, and over-voltage/current. i have no problem with full bridge, in fact i would welcome it. the reason i initialy picked half is cause i couldnt afford the igbts. now i have more than enough for 6 drsstcs in full bridge if i dont blow any.how about i post a schematic i plan on using, then u tell me what you think?(full bridge version)

Ben Solon

Fri Jul 01 2011, 05:25PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

here is the schematic, there are some problems(such as igbt symbol). does it matter where the decoupling cap is placed? the component values aren't set in stone. if you can, please post what the half bridge would look like, i have an idea, but you confused me.
thanks once again,
ben

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